For internal combustion engines, increasingly stringent statutory provisions make it necessary on the one hand to reduce as much as possible the raw emissions caused by the combustion of the air/fuel mixture in the relevant cylinder. On the other hand, exhaust after-treatment systems are employed in internal combustion engines to convert the pollutant emissions produced during the combustion process of the air/fuel mixture in the cylinder into harmless substances. Specifically in the case of gasoline engines a three-way catalytic converter located in the engine's exhaust tract is used for this purpose. A high degree of efficiency in the conversion of the pollutant components carbon monoxide, hydrocarbons and nitrogen oxides is only guaranteed in a very narrow window between the stoichiometric air/fuel ratio and approximately 6 tenths of a percent in the direction of excess fuel. To compensate for actual variations in the air/fuel ratio in the relevant cylinder, three-way catalytic converters are coated with a layer of material which can briefly store oxygen and bind or release it as required. Such a coating is known as a wash-coat and consists e.g. of Ce2O3 (dicerium trioxide). By means of this wash-coat, fluctuations in the air/fuel mixture and the corresponding exhaust gas are thus compensated in the catalytic converter as long as the wash-coat has not yet bound its maximum quantity of oxygen or else no more oxygen is bound in the wash-coat. However, if these limits are exceeded, the efficiency of the three-way catalytic converter is markedly reduced, resulting in increased pollutant emissions from the internal combustion engine.
In the case of closed-loop lambda control incorporating an oxygen sensor disposed upstream of the three-way catalytic converter, it is known to use the measurement signal of an oxygen probe which is disposed downstream of the three-way catalytic converter and generates a binary measurement signal, to adjust a P- or I-component of the control parameters or a delay time of the lambda controller accordingly as a function of the measurement signal of the oxygen sensor downstream of the three-way catalytic converter. This is also known as trim control. However, it has been found that, despite this measure, particularly in the case of aging three-way catalytic converters, undesirably high pollutant emissions of the internal combustion engine may continue to occur.
A method for operating a three-way catalytic converter is known from DE 101 03 772 A1, wherein said catalytic converter includes an oxygen-storing component which has a minimum and maximum filling level for oxygen. The three-way catalytic converter is disposed in an exhaust gas line of an internal combustion engine. The air/fuel mixture supplied to the engine is regulated in such a way that the filling level of the oxygen-storing component in the catalytic converter is kept within a mean setpoint range between the minimum and maximum filling levels. Drifting of the filling level out of the setpoint range is checked in a test phase in such a way that the filling level is increased or lowered relative to the instantaneous initial value by short-term reduction in richness or enrichment of the air/fuel mixture supplied to the engine by a certain amount and immediately returning to the initial value by a short-term opposing change in the air/fuel mixture. In the event of a breakthrough of lean or rich exhaust gas through the catalytic converter during the test phase, the air/fuel mixture is briefly enriched or reduced in richness in the form of a correction step in order to correct the air/fuel mixture supplied to the engine.
U.S. Pat. No. 6,253,542 B1 discloses an air-fuel mixture control in an internal combustion engine which comprises a post-catalyst exhaust gas sensor. If the measurement signal of the post-catalyst exhaust gas sensor is outside a predefined acceptable range, a proportional parameter of the control is set as a function of the measurement signal.
Methods for adjusting an air/fuel ratio in an internal combustion engine are also known from DE 100 28 570 A1 and DE 43 22 341 A1, said methods in each case having oxygen sensors which are disposed downstream of a catalytic converter.